1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, such as an isolator or circulator or the like used at high-frequency bands such as microwave bands, and to a communication device using the nonreciprocal circuit device.
2. Description of the Related Art
Conventionally, nonreciprocal circuit devices such as lumped parameter isolators and circulators have been widely used for communication devices and the like for ensuring stable operations and protection of oscillators and amplifiers, employing the properties thereof that the amount of attenuation in the sending direction of signals is extremely small and the amount of attenuation in the opposite direction is extremely great.
A exploded perspective view of a conventional isolator is illustrated in FIG. 7 and the internal structure thereof in FIG. 8. FIG. 9 illustrates an equivalent circuit.
As shown in FIGS. 7 and 8, the lumped constant isolator is arranged, within a magnetic closed circuit made up of an upper yoke 2 and lower yoke 8, a magnetic assembly 5 comprising central conductors 51, 52, and 53, and ferrite 54, and a permanent magnet 3 and a resin case 7. The port portions P1 and P2 of the central conductors 51 and 52 are connected to input/output terminals 71 and 72 formed within the resin case 7, and matching capacitors C1 and C2, the port portion P3 of the central conductor 53 is connected to the matching capacitor C3 and a terminal resistor R, and the matching capacitors C1, C2, and C3 and the terminal resistor R at one end are connected to the ground 73.
In the equivalent circuit shown in FIG. 9, the ferrite is represented having the shape of a disc, the DC magnetic field is denoted by H, and the central conductors 51, 52, and 53 are represented as equivalent inductors L. Due to such a circuit configuration, the forward direction properties have band-pass filter properties, and even signals in the forward direction are somewhat attenuated at frequency bands farther away from the pass band.
Now, in normal communication devices, amplifiers used in the circuit always generate a certain level of distortion, which generates spurious waves such as second and third harmonic frequencies of the fundamental frequency, leading to unwanted radiation. Unwanted radiation of communication devices lead to abnormal actions of electric power amplifiers and interference, so standards and stipulations are provided beforehand, and the radiation must be kept under a certain level. Using amplifiers with good linear properties is effective for preventing unwanted radiation, but these are expensive, so filters are generally used instead to cause attenuation of unnecessary frequency components. However, such filters also increase costs and increase the device size, and further, there is loss due to the filters.
Thus, an arrangement can be conceived wherein the band-pass filter properties of isolators and circulators are used to suppress spurious components, but with a nonreciprocal circuit device having only the basic conventional configuration shown in FIGS. 7 through 9, sufficient attenuation characteristics have not been obtained at unnecessary frequency bandwidths.
A nonreciprocal circuit device which solves this problem and enables great attenuation amounts to be obtained at spurious frequency bandwidths such as twofold or threefold waves of the basic wave is disclosed in Japanese Unexamined Patent Application Publication No. 10-93308. An isolator which is an example of this nonreciprocal circuit device is shown in FIGS. 10 through 12. FIG. 10 is a exploded perspective view of the isolator, FIG. 11 is the internal structure thereof, and FIG. 12 is an equivalent circuit.
This isolator differs from the isolator described above with reference to FIGS. 7 through 9 in that an inductor Lf is provided for a band-pass filter. This inductor Lf is connected between the port portion P1 of the central conductor 51 and the matching capacitor C1 and the input/output terminal 71. A solenoid coil suitable for miniaturization is used for the inductor, and in the case of a 900 MHz band isolator, an item with approximately 24 nH inductance is used. Specifically, an article wherein a copper wire having a diameter of 0.1 mm is wound nine turns on an external diameter of 0.8 mm, is used.
Serially connecting a capacitor Cf to the input/output terminal 71 of an isolator, a band-bass filter is formed by the capacitor Cf and the inductor Lf as shown in FIG. 12, so signals of frequencies away from the pass band can be attenuated.
FIG. 13 is a diagram illustrating the frequency properties of the isolator shown in FIGS. 7 through 9 (first conventional example) and the isolator for a 900 MHz band, and it can be understood that the second harmonic frequency (1800 MHz) attenuation amount has been improved from 19.3 dB to 28.3 dB in comparing the second conventional example with the first conventional example, and the third harmonic frequency (2700 MHz) attenuation amount has been improved from 28.6 dB to 40.1 dB.
Thus, a filter for attenuating unnecessary frequency bandwidth constituted by providing an inductor within a nonreciprocal circuit device allows the overall circuit to be reduced in size as compared with providing an individual filter outside.
However, the recent demand for further miniaturization of mobile communication devices is necessitating further reduction in size of nonreciprocal circuit devices with inductors serving as filters, and accordingly, such inductors serving as filters must be reduced in size as well. However, in the event of reducing the size of inductors formed as solenoids, the inductance thereof becomes small, and the amount of attenuation at twofold and threefold waves of the basic wave becomes small. Also, a structure wherein a solenoid is formed within the magnetic body in order to reduce the size of the solenoid-shaped inductor without loosing inductance has been conceived, but such a structure requires a new magnetic body, of which the manufacturing process is not easy, and this is problematic since it would lead to increased costs.